During construction of a well penetrating a subterranean formation, a rotary drill is typically used to bore through the subterranean formation to form a wellbore. Once the wellbore has been drilled, a pipe or casing is lowered into the wellbore. A cementitious slurry and a displacing fluid, such as a drilling mud or water, is pumped down the inside of the pipe or casing and back up the outside of the pipe or casing through the annular space between the exterior of the pipe or casing and the wellbore. The cementitious slurry is then allowed to set and harden.
A primary function of the cementing process is to restrict fluid movement between the subterranean formation and to bond and support the casing. In addition, the cement aids in protecting the casing from corrosion, preventing blowouts by quickly sealing formations, protecting the casing from shock loads in drilling deeper wells, sealing off lost circulation or thief zones and forming a plug in a well to be abandoned. Cementing operations further provide zonal isolation of the subterranean formation and help prevent sloughing or erosion of the wellbore. In addition to their use in oil gas wells, cementitious slurries may be used to cement pipes or casings within geothermal wells, water wells, injection wells, disposal wells and storage wells.
In addition to selectively isolating particular areas of the wellbore from other areas of the wellbore, cementitious slurries may further be used for other purposes. For instance, cements may be used in remedial operations to repair casing and/or to achieve formation isolation as well as in sealing off perforations, repairing casing leak/s (including leaks from damaged areas of the casing), plugging back or sealing off the lower section of a wellbore, etc.
Cementitious slurries for use in such applications contain hydraulically active cements which set and develop compressive strength due to a hydration reaction. Physical properties of the set cement relate to the x-ray amorphous structure of the calcium-silicate-hydrates formed during hydration. For example, conventional Portland cements form an interlocking network of, for example, tricalcium silicate, dicalcium silicate, tetracalcium aluminum ferrite hydrates, interspersed with calcium sulfate and calcium hydroxide crystals. These crystals interconnect to form an interlocking structure which provides both flexural strength and a degree of resiliency.
Gas channeling in a cement composition is a common problem in the oil and gas industry. When a cement slurry is first placed in the annulus of an oil or gas well, it is the hydraulic fluid that exerts hydrostatic pressure on the sides of the well. Initially the hydrostatic pressure of the cement composition is great enough to keep gases that are naturally occurring within the reservoir in situ. But as the slurry of cement composition sets, it goes through a transition stage changing from liquid to solid. During this transition stage, the cement composition exerts less and less hydrostatic pressure on the well. It is in this transition stage that the cement composition is susceptible to formation gas entering into the cement sheath. The gas entering into the cement sheath produces pathways filled with gas. As the cement hardens, the pathways become channels in the hardened cement composition. Channeling in a cement composition weakens the structure.
Another common problem in well cementing is the loss of liquid fluid from the cementitious slurry into porous low pressure zones in the formation surrounding the well annulus. Fluid (liquid and/or gas) loss is undesirable since it can result in dehydration of the cementitious slurry. In addition, it may cause the formation of thick filter cakes of cement solids. Such filter cakes may plug the wellbore. In addition, fluid loss can damage sensitive formations. Minimal fluid loss is desired therefore in order to provide better zonal isolation and to minimize formation damage by fluid invasion.
Controlling gas in light weight cements, especially at low temperatures, has also been an industry problem for a number of years because the additive systems that are generally used or employed are better suited for heavier or higher density cements.
Common additives used to control fluid loss and gas migration from the slurry to the porous permeable formation include hydroxyethyl cellulose (HEC), carboxymethylhydroxyethyl cellulose (CMHEC), acrylamidomethylpropane sulfonic acid (AMPS), polyethyleneimines, styrene butadiene rubber latexes and polyvinyl alcohol. Further, microparticulate additives, such as silica fume, may be used in combination with such additives to make the cement composition less permeable. Such materials work best, however, in cement compositions that have a high cement density and a low water to cement ratio. The lower the cement density and the higher water to cement ratio, the greater the quantity of water soluble or film-forming additives that are required to reduce gas migration to an acceptable level and keep channeling to a minimum. The lower the cement density, therefore, the greater the quantity of traditional additives that are required. This quantity increases to a point that is cost prohibitive for lower density cement compositions.
Alternative additives for controlling fluid loss and gas migration have therefore been sought.